Neutron detection is conventionally accomplished by using a detector of ionizing radiation and employing a conversion nuclear reaction whereby the neutron produces a charged particle product. The first neutron detectors were gas proportional counters which detected ionization produced by the highly charged fission fragments produced in neutron induced fission of .sup.235 U.
An alternative to using a gas-filled detector is to use a semiconductor or solid state detector. Conventional semiconductor neutron detectors consist of a silicon surface barrier detector with a layer of boron, lithium or fissionable material adjacent to the active volume of the detector. One such solid state neutron detector using a silicon semiconductor is disclosed in U.S. Pat. No. 3,227,876.
A problem with prior art neutron detectors is sensitivity of the detector to non-neutronic components of the radiation field, particularly gamma ray sensitivity. Gas-filled detectors are favored in nuclear reactor applications because low density gases are inherently inefficient detectors for gamma rays which deposit their energy over large volumes. Solid-state detectors, on the other hand, are more sensitive to gamma rays because of their higher electron density.
Another problem with conventional neutron detectors used in nuclear reactors is their inability to operate over the entire power range of the reactor. Different types of gas-filled detectors are currently used during start up (&lt;1 n/cm.sup.2 /sec), ramp up (10.sup.3 -10.sup.7 n/cm.sup.2 /sec), and full power (10.sup.9 -10.sup.11 n/cm.sup.2 /sec) of the nuclear reactor.
Solid-state semiconductor detectors are candidates for replacement of conventional gas-filled detectors, but they have not found widespread use in the nuclear industry because of problems associated with background signal and deterioration of detector performance during operation in intense, hostile radiation environments. The detector assembly of the present invention eliminates or reduces many of the problems formerly associated with gas-filled and solid-state electronic detectors.